Metabolic processes are typically highly susceptible to even slight changes in pH, and therefore, proper control of this parameter is critical. Precise manipulation of pH can determine the relative yield of the desired species over competing by-products. Deviations of as little as 0.2 to 0.3 may adversely affect a batch in some cases. Like the cell mass probe and dissolved oxygen probes described earlier, the pH probe (see Fig. 5) is packaged in a sterilizible inert casing with permeable electrode facings for direct insertion into the bioreactor. The measurement principle is the oxidation reduction potential of the hydrogen ion and the electrode materials are selected for that purpose.
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